1. Field of the Invention
The present invention relates generally to the field of radiation sensitive resist films used in microcircuit fabrication and, more particularly, to a dry plasma developable electron resist process for developing negative electron resists.
2. Description of the Prior Art
The application of electron beam techniques to the art of semiconductor fabrication has enabled great strides to be made in reducing the minimum line width and thus the size of a pattern which can successfully be manufactured. This has been accomplished both through progress in the technology involved with the perfection in the precision of the electron beam system itself and in the progress which has been made in the technology concerned with the pattern defining medium or resist material.
In electron beam microfabrication, a substrate, which may be, for example, silicon dioxide, silicon, glass or chromium plated glass, is further coated with a layer of polymer resist material and the resist is patterned by changing the solubility of the polymer with an electron beam. Subsequently, in accordance with prior art processes, the mask has heretofor been "developed" by dissolving away the unwanted area of polymer utilizing a suitable solvent material and the resultant pattern used as a mask either for plating, chemical etching, ion etching or ion implantation. Electron-beam lithography (EBL), then, is an integrated circuit production technique which utilizes a polymer resist to delineate circuit patterns for monolithic circuits.
Insofar as the solubility of the polymers is concerned, when polymers of the required type are irradiated with an electron beam, the molecular structure is affected such that some of the polymer molecules are excited or ionized by the beam. This excitation causes some of the resist molecules to cross-link with other molecules in the polymer structure and others to degrade or undergo scission. The predominant manner in which such a polymeric material reacts to exposure to an electron beam has led electron resists to be classified into two main categories. Thus, a polymer which becomes predominantly gelled or cross-linked, and thereby decreases its solubility after irradiation, is termed a negative resist. Conversely, if the scission process predominates and the solubility of the polymer increases after irradiation, it is called a positive resist. The resists developed by the process of the present invention are negative resists.
A suitable electron resist must have various physical and chemical properties which allow it to fulfill the requirements for electron beam fabrication. The polymer material involved must be sensitive to an electron current of a fairly low value or the resist sensitivity itself will be the limiting factor on the writing speed and line width which can be achieved. The resist medium must be capable of a high resolution or resist contrast compatible with that achieved in writing and etching techniques utilizing the high resolution capability available with electron beam technology. The resist must also have the ability to adhere satisfactorily to a variety of substrates used in different microfabricated applications. The medium also must be able to withstand normal acid, base, and plasma and ion etching processes and should not be sensitive to small daily process variabilities.
Negative electron resists are normally applied to the substrate by a spinning process, and, as previously stated, exposed using a high energy electron beam and spray developed with a suitable solvent. It is the solvent development step in the process which has been somewhat of a limiting factor in microcircuit fabrication utilizing this technique. Since the unexposed area has to be removed completely by the solvent, the solvent invariably penetrates into the exposed area to a certain extent thereby causing the swelling of the cross-linked polymer. The swelling of negative resist polymers associated with wet solvent development is a serious problem and has limited the degree of high resolution obtainable in negative resist pattern technology. In addition, the solvents utilized in the development are often quite toxic to those working with them and all are relatively costly.
In the prior art, dry process utilizing a gas plasma, preferably an oxygen plasma has been disclosed in the development of photoresist type films. One such process is illustrated and described in U.S. Pat. No. 4,241,165 to Hughes et al Dec. 23, 1980. Such photoresist material normally consists of a polymer binder, a vinyl aromatic monomer and a photoinitiator which, upon exposure to ultraviolet light decompose and initiate a polymerization reaction. While the process of that invention appears to work satisfactorily for such photoresist materials, the electron beam fabrication technique requires different material properties which makes photoresist materials unsuitable.
Dry process developable photoresists are not suitable for use in electron resist processes because the vinyl aromatic monomers which exist in the solid state at room temperature sublime or evaporate under the high vacuum required for electron beam lithography. To overcome this problem, electron resist materials normally must be further coated with a layer of barrier polymer to prevent sublimation of the resist material. In addition, resolution required for modern day microcircuit fabrication which is normally a resolution of less than one micron cannot be obtained utilizing photoresist materials. Thus, photoinitiated polymerization cannot be used because of optical limitations.